1) Field of the Invention
The present invention relates to an optical fiber applied to an optical amplifier suitable for optical communications and other related applications.
2) Description of the Related Art
In the field of recent optical communication systems, research and development in wavelength division multiplexing (WDM) transmission and building of optical communication networks using the WDM technology is now in progress. The WDM technology is inevitable for developing a large-capacity communication system.
An erbium-doped fiber (EDF), which is manufactured by doping element erbium (Er) into a core of a silica glass optical fiber, has an amplification band in a wavelength band of 1.55 micrometers that is the same as lowest loss band of the silica glass optical fiber. Furthermore, the EDF can take almost the same material and the same structure as an optical fiber for transmission, which makes it possible to realize a low connection loss when connecting with the optical fiber for transmission. For these reasons, the EDF is widely used as an optical amplification fiber for amplifying an optical signal in an optical transmission system.
An erbium-doped optical fiber amplifier (EDFA) employing the EDF has a high efficiency and a high gain, which can maintain a high-density pumping light and a signal light while overlapping the both in a long-haul transmission. The EDFA plays an important role as a key device in the WDM transmission system.
With the recent establishment of the WDM transmission technology, an introduction of the WDM transmission system became active in a long-haul large-capacity optical communication system, and a transmission capacity shows a continuing increase. Regarding such a WDM transmission, an experimental result of a transmission speed over terabit has been reported. When applying an optical amplification fiber, such as the EDF, in the long-haul high-speed large-capacity optical communication network, a high-quality optical amplification fiber is required.
One of the important parameters of a quality of amplification in the optical amplification fiber is the noise figure (NF). The noise characteristic of the optical amplification fiber is evaluated based on the NF.
One of the methods to reduce the NF is, as described in U.S. Pat. No. 6,804,046, to increase a ratio of an absorption coefficient αp in a pumping light wavelength band to an absorption coefficient αs in a signal light wavelength band. When the ratio αp/αs is increased, the ratio of the population inversion increases whereby the NF is decreased.
One of the refractive index profiles of a general optical amplification fiber is a step-index type, as shown in FIG. 3 and FIG. 4. Regarding an area for doping a rare-earth element, a step-index type structure in which the rare-earth element is doped in an entire core 1 shown in. FIG. 3 (hereinafter, “step-index type”) and a center-doped type structure in which the rare-earth element is doped only in a center part of a core 1 shown in FIG. 4 (hereinafter, “center-doped type”) are popular. The hatched portions in FIG. 3 and FIG. 4 indicate areas in which the rare-earth element is doped. In an explanation for FIG. 3, FIG. 4, and FIG. 5, a description for an outer portion of a cladding 4 (specifically a third cladding 4 in FIG. 5) is omitted. In the technology disclosed in Japanese Patent Application Laid-Open No. 2002-151772, the center-doped type is mainly used.
However, in the conventional method disclosed in U.S. Pat. No. 6,804,046, it is necessary to narrow a rare-earth doped area in the center-doped type to increase the value of αp/αs. When the rare-earth doping area is narrowed, the absorption coefficient of the optical amplification fiber (absorption coefficient per unit length) becomes small. As a result, a length of the optical amplification fiber used must be longer to compensate for a decrease in the absorption coefficient.
Furthermore, in the center-doped type, it is difficult to match refractive indexes between the rare-earth doped area and a non-doped area in the core, resulting in a manufacturing problem.